Conventional communications transmitters employ a quadrature modulator to modulate information to be transmitted, such as voice or data, onto a radio frequency (RF) carrier signal that is capable of being transmitted through the atmosphere to a remote receiver.
FIG. 1 is a simplified drawing of a conventional quadrature-modulator-based transmitter 100. The quadrature-modulator-based transmitter 100 comprises a quadrature modulator 102 and a power amplifier (PA) 104. The quadrature modulator 102 includes in-phase (I) and quadrature phase (Q) mixers 106 and 108, a local oscillator (LO) 110, a ninety-degree phase shifter 112 and a summer 114.
The I mixer 106 operates to modulate an I signal onto a radio frequency (RF) carrier signal generated by the LO 110 while the Q mixer 108 operates to modulate a Q signal onto a ninety-degree phase-shifted version of the RF carrier signal. The upconverted I and Q signals are summed by the summer 114 and typically filtered by a band-pass filter (not shown) to create a filtered modulated RF carrier signal. The PA 104 amplifies the band-pass-filtered modulated RF carrier signal to produce the desired modulated RF output signal RFout.
One desirable characteristic of the quadrature-modulator-based transmitter 100 is that the frequency and phase of the RF carrier signal can be modulated simply by manipulating the amplitudes of the I and Q signals. However, a significant limitation is that it is not very power efficient, particularly for communication technologies that employ nonconstant-envelope signals, such as orthogonal frequency division multiplexing (OFDM), and other existing or soon-to-be deployed cellular technologies, such as W-CDMA, High-Speed Packet Access (HSPA) and Long Term Evolution (LTE) communication technologies. To prevent clipping of the signal peaks of these nonconstant-envelope signals in the quadrature-modulator-based transmitter 100, the signal levels must be reduced before being introduced to the PA 104, and the PA 104 must be configured to operate in its linear region of operation. Unfortunately, linear PAs configured to operate at reduced drive levels are not very power efficient. This lack of power efficiency is a major concern, particularly in battery-powered applications such as, for example, cellular handsets.
One known way of avoiding the linearity versus power efficiency trade-off of the quadrature-modulator-based transmitter 100 is to employ an alternative type of communications transmitter known as a polar modulation transmitter (also commonly referred to as an envelope-elimination and restoration (EER) transmitter). FIG. 2 is a drawing of a typical polar modulation transmitter 200. The polar modulation transmitter 200 comprises a CORDIC (Coordinate Rotation Digital Computer) converter 202, an amplitude modulator 204 configured in an amplitude path, a phase modulator 206 configured in a phase path, and a PA 208.
During operation, the CORDIC converter 202 converts the rectangular-coordinate I and Q signals into polar-coordinate amplitude and phase component signals ρ and θ. The amplitude modulator 204 modulates a direct current (DC) power supply Vsupply according to amplitude variations in the amplitude component signal ρ. The resulting amplitude modulated power supply signal Vs(t) is coupled to the power supply port of the PA 208. Meanwhile, the phase modulator 206 modulates an RF carrier signal in accordance with phase information contained in the phase component signal θ. The resulting phase-modulated RF carrier signal RFin is coupled to the RF input RFin of the PA 208. Because the phase modulated RF carrier signal RFin has a constant envelope, the PA 208 can be configured to operate in its nonlinear region of operation, where it is efficient at converting DC power from the DC power supply Vsupply to RF power at the output of the PA 208. Typically the PA 208 is implemented as a Class D, E or F switch-mode PA 208 operating in compression, so that the output power of the PA 208 is directly controlled by the amplitude modulated power supply signal Vs(t) applied to the power supply port of the PA 208. Effectively, the PA 208 operates as a multiplier, amplifying the constant-envelope phase modulated RF carrier signal according to amplitude variations in the amplitude modulated power supply signal Vs(t), to produce the desired amplitude and phase modulated RF carrier signal RFout.
Although the polar modulation transmitter 200 is significantly more power efficient than the quadrature-modulator-based transmitter 100, it has various drawbacks of its own. First, converting the I and Q signals from rectangular to polar-coordinates often results in substantial bandwidth expansion. Bandwidth expansion is a major concern since the rate at which the polar modulation transmitter's digital signal processing circuitry must process the amplitude and phase component signals ρ and θ is determined by the bandwidths of the signals. The wider the bandwidths are, the faster the processing rates must be.
The degree to which the bandwidth expands in the rectangular-to-polar conversion process is largely dependent on the modulation format being employed. Nonconstant-envelope modulation formats that have signal trajectories which pass through, or close to, the origin in the I-Q signal plane result in the most severe bandwidth expansion. In fact, a signal trajectory that does pass through the origin results in an instantaneous phase shift of 180 degrees. Not only are such rapid changes in phase difficult to digitally process, they are also difficult to translate to RF. For example, in the phase path, the phase modulator 206, which is responsible for modulating the phase component signal θ onto the RF carrier signal, is capable of providing a linear response only over a narrowly-defined frequency range and, therefore, is unable to react to abrupt changes in the phase of the phase component signal θ. Bandwidth expansion of signals in the amplitude path can also be problematic. Typically the amplitude modulator 204 comprises a switch-mode converter of some sort. Accurate tracking of the amplitude component signal ρ requires that the switching frequency be about twenty to fifty times higher than the signal envelope bandwidth. Given that the switching transistors in state-of-the-art switch-mode converters can only be switched up to a maximum of about 5 MHz, the bandwidth of the amplitude component signal ρ is often a problem that must be addressed.
Another drawback of the polar modulation transmitter 200 involves the timing of the amplitude and phase component signals ρ and θ. Because the amplitude and phase component signals ρ and θ are processed in different paths using different components and at different processing rates, a delay mismatch typically arises between the signals at the RF input and supply ports of the PA 208. This delay mismatch results in spectral regrowth, a highly undesirable condition that impairs the ability of the polar modulation transmitter 200 to comply with out-of-band noise limitation standards.
Considering the foregoing limitations and drawbacks of existing communications transmitters, it would be desirable to have methods and apparatus for transmitting communications signals that are not only power efficient but which also avoid the difficulties of processing wide bandwidth signals.